SONAR (SOund Navigation And Ranging) is the generic name of the technology that is used to locate objects underwater. SONAR is used in marine, geological, and biological research, undersea mapping and navigation, and various commercial and military applications.
An “active” SONAR system is a type of SONAR system in which a “projector” emits a pulse of sound and underwater microphones called “hydrophones” receive underwater sounds to be signal processed. If the transmitted pulses encounter an underwater object (a “target”), a portion of the sound is reflected as an “echo.” Knowing the speed of sound in water and the time for the sound wave to travel to the target and back, the distance between the listening-post vessel (e.g., ship, etc.) and the target can be calculated. Active sonar systems generally use highly directional beams of sound when searching for targets, which enable them to determine direction to the target, as well as the distance.
Another application of active SONAR processing is for measuring the velocity of the sound-projecting vessel itself. The SONAR source of the vessel directs sonic pulses towards the ocean floor, and the receivers detect echoes of those pulses. The velocity of the vessel is then calculated based upon the distance traveled by the vessel between the transmission and reception of a first pulse and a second pulse. Examples of velocity-measurement SONARs are spatial correlation SONAR and temporal correlation SONAR, which rely on selecting a maximum “correlation” between hydrophones in the case of spatial correlation or pulses in the case of temporal correlation.
Although hydrophones can be used singly, they are often used in an array. A hydrophone array is made up of a plurality of hydrophones that are placed in known locations. For example, hydrophones can be placed in a line on the seafloor, moored in a vertical line in the water column, or towed in a horizontal line behind a ship or submarine.
A hydrophone array is much better at detecting a single specific sound than a single hydrophone. This is because the array is able to filter out noise coming in from all directions and focus on sounds arriving from a specific direction. The increased signal-to-noise ratio allows sounds that normally could not be detected by a single hydrophone to be heard. Furthermore, if a hydrophone array is being used to receive a specific sound source, the source can be quieter, yet still be detected.
Although specifics can vary depending upon the algorithm used, a hydrophone array determines the direction of the source of a sound in the following manner. Consider a sound arriving at a hydrophone array from a distant source, such as a submarine. The sound will reach each hydrophone in the array at slightly different times based on their different positions in the array and as a function of the direction from which the sound is coming. This time difference is known as the time-of-arrival-difference. Using this information from all the hydrophones in the array, and knowing the specific location of each hydrophone in the array, the direction from which the sound is coming can be determined.
It will be appreciated that if one or more of the hydrophones in a hydrophone array malfunction, the malfunction can degrade the performance of the associated SONAR system. While the performance degradation issue can be addressed, the more vexing issue is detection. That is, how does one detect if a hydrophone is malfunctioning or has failed? This is a critical issue because if undetected, a malfunctioning hydrophone can result in inaccuracies in the solutions obtained from the SONAR system. Even worse, the inaccuracies might not be recognized as such.
In order for SONAR systems to provide reliable information, the hydrophones that provide the SONAR data must be reliable. Determining the reliability of the hydrophones requires that hydrophone channel health tests be performed. A “hydrophone channel” is defined herein as including a hydrophone and all associated cabling, signal routing, and processing of that hydrophone's output.
Various techniques in the prior art exist for testing the hydrophone channels. Those techniques include a “low-noise” test, a “reverberation” test, and a “high-noise” test. The low-noise test is designed to detect a low noise level in a situation where a higher noise level is normally expected. Disadvantageously, this test only works properly in a high-noise environment and is susceptible to variable sea noise effects. The reverberation test is designed to detect an echo from close-in, own-transmission reverberation off the water volume and then to compare the level of the detected echo against a predetermined threshold; the test declares the associated hydrophone channel as having failed if the compared level is insufficient. However, this test is only designed to detect a low signal, not noise intrusion, and it might not detect a failing hydrophone. The high-noise test is designed to detect a high noise level. But as with the low-noise test, this test's shortcoming is that it is also susceptible to variable sea noise effects.
What is needed is an improved technique for performing fault detection on hydrophone channels, without some of the disadvantages in the prior art.